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One of the call differences I have needed to learn to distinguish is the "dyspeptic groans" of the two species of Spadefoot Toads (genus Scaphiopus) that occur in my area.Hurter's Spadefoot Toad (Scaphiopus hurterii) is closely related to the Eastern Spadefoot Toad (Scaphiopus holbrooki) and occupies the eastern half of Texas. It is usually greenish or brownish with vague striped pattern on its back.

The widespread Spadefoot of the southwestern US is the Couch's Spadefoot (Scaphiopus couchii). In Texas, it is found in the western 2/3 of the state. Couch's Spadefoot is highly variable in pattern, but usually shows some pattern of a yellowish or greenish background with an irregular mottled or spotted black pattern on the back. It rarely shows the striping seen in S. hurterii.

Not only is there a superficial resemblance in size and coloration, but these two species have similar calls. Their calls are often described as "dyspeptic groans" and can sound similar from a distance.

Here is the call of Scaphiopus hurterii. This species has a "doppler" effect to its call where its shorter notes descend in pitch. It is somewhat remniscent of impersonating the sound of a car passing at high speed.

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They don't sound that similar up close, but from a distance they can be suggestive of each other. But while they are similar sounding, the "shape" of the call notes and their length is different. Here is a few seconds of S. hurterii followed by a few seconds of S. couchii.

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This difference is relatively easy to see when you look at the spectrograph of their call notes. Here is the spectrograph of a single S. hurterii call followed by a single S. couchii call.

The darkest line on the spectrograph represents the carrier (dominant) frequency of the call. I have drawn in a red line indicating the approximate shape (slope) of the call in an effort to demontrate the differences in their relative shapes.Here's a comparison of a chorus of S. hurteri from DeWitt County, Texas -
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Hurter's Spadefoot Chorus - DeWitt Countyand a chorus of S. couchii from Dimmitt County, Texas -
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Couch's Spadefoot Chorus - Dimmit County